Ion generator having beam stabilization accelrating electrodes



Se t. 5, 1967 (5. ca. KELLEY 3,340,425

ION GENERATOR HAVING BEAM STABILIZATION ACCELERATING ELECTRODES Filed Aug. 31, 1966 3 Sheets-Sheet 1 v I r 7 I. v v E;

I INVENTORQ George G. Kelley ATTORNEY.

Sept. 5, 1967' G. G. KELLEY 3,340,425

ION GENERATOR HAVING BEAM STABILIZATION ACCELERATING ELECTRODES Filed Aug. 51, 1966 3 heets-Sheet 2 INVENTOR. George G. Kelley ATTORNEY.

P 1967 G. G. KELLEY $340,425

ION GENERATOR HAVING BEAM STABILIZATION ACCELERATING ELECTRODES Filed Aug. 31, 1966 3 Sheets-Sheet 3 V. O z

Y I 2 1 I b I i c I v 1 I T I l I J I v Fig.4

INVENTOR. George G. Kelley B). v

ATTORNE Y.

3,340 425 ION GENERATOR HAVI NG BEAM STABILIZA- TION ACCELERATING ELECTRODES George G. Kelley, Kingston, Tenn., assignor to the United States of America as represented by the United States Atomic Energy Commission Filed Aug. 31, 1966, Ser. No. 576,791 6 Claims. (Cl. 315-111) ABSTRACT OF THE DISCLOSURE voltages and higher magnetic fields while at the same time providing an ion beam that is well focused, of good quality, and with an increased output.

The invention described herein was made in the course of, or under, a contract with the US. Atomic Energy Commission.

Ion accelerators are utilized in several research programs at various government laboratories. For example, such ion accelerators are used for the injection of ions into various of the controlled thermonuclear reactor devices and for various accelerator systems. The ion accelerator of the present invention is an improvement over the ion accelerator region of the ion source described in US. Patent No. 3,238,414, issued Mar. 1, 1966, to George G. Kelley et al. The use of electron dump fins in the region between the anode and the extracting electrode in the above prior patent to minimize the severe sparking in that region caused by oscillating electrons which caused such sparking in ion accelerators prior to that patent, the use of a plasma epxansion cup as part of the anode structure in that patent, and providing a solenoid focusing magnet in conjunction with a smaller solenoidal magnet encompassing the above plasma expansion cup in that patent to provide a magnetic field nearly zero at the exit of the cup all contributed to providing an output ion beam with a substantial improvement in quantity and quality thereof as compared to prior devices. The term quality is defined in the above prior patent, to which reference is made.

The device of the above prior patent, although substantially better than those available therefore, was still inadequate in the sense that, when operated at 100 kv, the focusing magnetic field must be less than that desired in order to provide for optimum focusing of the ion beam; otherwise, sparking is excessive even with the use of the electron dump fins when attempts are made to increase the strength of the magnetic field above that which is necessary for efiicient operation and focusing of the ion beam. This problem is accentuated further when even higher voltages are desired such as for high voltage (600 kv.) acceleration of the ion beam from the device.

With a knowledge of the limitations of the prior ion accelerators, as discussed above, it is the object of the present invention to provide an improved ion accelerator wherein the ion accelerating electrodes thereof may be operated at higher voltages and the focusing magnetic fields may be more intense than heretofore possible, while at the same time providing an ion beam that is well focused, of good quality, and that has an improved output.

This and other objects and advantages of the present invention will become apparent upon a consideration of United States Patent Patented Sept. 5, 1967 the following detailed specification and the accompanying drawings, wherein:

FIG. 1 is a partial sectional view of the device of the above prior patent illustrating the relationship between the equipotential planes and magnetic flux lines of the accelerating region thereof;

FIG. 2 is a partial sectional view of an ion source embodying the present invention illustrating the relationship between the equipotential planes and magnetic flux lines of the accelerating region thereof;

FIG. 3 is a cross-sectional view of a complete ion source employing the present invention; and

FIG. 4 is a partial sectional view of an ion source wherein the present invention is utilized in a multi-accelerating electrode design.

The above object has been accomplished in the present invention by providing a rearrangement of the accelerating electrodes in the accelerating region of the prior device such that there is -no point of maximum potential along a magnettic field line, and electron oscillation cannot occur and thus the electrons are drawn out of the region as they are formed to thereby prevent accumulation and uncontrolled discharge therefrom.

In the drawings, FIG. 1 illustrates the relationship between equipotential planes, shown as solid lines, and magnetic field lines, only one of which is shown as a dashed line, in the accelerating region of the ion source device of the above prior patent. It should be noted that the relative positions of components, and the shape of the equipotential planes and magnetic field line are not necessarily exact; however, there is a point of maximum electrical potential that exists along the field lines, as shown. Thus, an electron generated at point A moves along the field line B since there is a more positive potential at point C. The electron then oscillates along the flux line near the point C. Other electrons that are generated along the flux line B as well as electrons that are generated along other flux lines, not shown, will oscillate in the same manner and permit electron buildup which ultimately results in an undesirable electrical discharge or discharges especially at higher accelerating voltages and magnetic fields even with the use of the electron dump fins, not shown, since the fins do not remove all of the oscillating electrons at the higher operating values of accelerating voltages and magnetic fields.

If, however, an accelerating gap is provided, as in the present invention, such that there are no points of maximum potential along magnetic field lines, electron oscillation cannot occur and thus the electrons are drawn out of the accelerating region as they are formed to thereby prevent accumulation and uncontrolled discharges. Such a gap design is shown in FIG. 2. Here the equipotential planes and magnetic field lines produce the desired conditions. Thus, an electron at point A can move along magnetic field line B, influenced by the potential gradient, without encountering a point of maximum potential prior to reaching the positive potential electrode. With such a gap design as shown in FIG. 2, it has been determined that there is no need for electron dump fins in the accelerating region and the ion accelerator can be operated at higher accelerating voltages and magnetic focusing fields than heretofore possible to producea stable, well focused ion beam.

In FIG. 1 and FIG. 2, only the components of the accelerating region are shown, such as the source housings 1, 1; the anodes 2, 2'; the plasma expansion cups 3, 3'; the solenoidal magnets 4, 4; and the accelerating electrodes 5, 5'.

FIG. 3 illustrates the actual construction of an ion source embodying the present invention. The components 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 21, 22, and 23 to the cup 21.

the form of an inverted cup, is

' position, as shown,

- equipment, not shown.

- tion of the feed gas.

3 of FIG. 3 are substantially the same and function in the same manner as the corresponding components in the above-mentioned prior Patent No. 3,238,414. The accelerating region of FIG. 3, however, is different from that utilized in the above patent and corresponds to that described above for FIG. 2. For example, a base'plate 36 of the source unit rests in a recess of an extensive fiat plate 37 so that together they form a flat electrode of the accelerating system. A second electrode 24, in the form of an inverted shallow dish, is positioned in proper rela- 'tionship to give the desired potential gradient with respect The ion aperture of the electrode 24 is defined'by a graphite insert 25 so as to withstand operating temperatures. The walls of the dish-like electrode 24 are supported from a flat insulator 19. The plate 37 is also supported from this insulator. A third electrode 34, in

provided as shown such that the electrodes 24 and 34 provide an accel-decel combination that traps electrons between these electrodes and the magnetic lens 7 and allows space charge neutralization in this region. It should be noted that the electrodes 24 and 34 have the same relative configuration with respect to the flat electrode formed by members 36, 37 whether the device of FIG. 3 is operated in a vertical or is operated in any other position for example.

The exact shape of the electrode 24 is not critical except that the resultant equipotential planes which are formed permit proper electron drain and that no portion of the insulator 19 can see any of the ion beam exiting from the cup 21 or through the aperture in the insert 25 of the electrode 24. Several apertures 26 are provided in the side walls of the electrode 24 in alignment with slits 27 in the insulator 19 to improve the pumping by vacuum Thus, a bafiie 20 is provided to block the direct line of sight from an ion beam to the aforementioned insulator 19.

The cathode 9 of FIG. -3 is connected to an adjustable source of filament supply (0-40 amperes), not shown, over leads 14. The source magnet coil 8, when used, is connected to an adjustable source of supply (0-3 amperes), not shown, over leads 13, An adjustable arc supply voltage (0-300 volts), not shown, is connected over leads 15 between the cathode 9 and the anode 22. The intermediate electrode member 10 is connected to a source of supply, not shown, which is about 30 volts positive with respect to the cathode 9, for example. An adjustable source of accelerating voltage (0-100 kv.), not

"connected to ground. The water cooling tubes 11 and 12 are provided to cool the intermediate electrode 10 and V the anode 22, respectively.

Gas from a gas source, not shown, is fed at a controlled rate into the chamber between the intermediate electrode 10 and the anode 22 by means of a feed tube 16 with an 0.1-inch opening. Alternatively, gas may be fedinto the cathode chamber, if desired, for better utiliza- The coil 23 encompassing the plasma expansion cup 21 is connected to an adjustable source of supply (0-300 amperes), not shown.

It should be noted that the device of FIG. 3 may be operated with or without current to the coil 8, depending upon whether a focused or non-focusedion beam is 'desired from the device in the same manner as in the above-mentioned prior patent. However, in all operations, the current to the coil 23 is adjusted either alone or in combination with adjustment of the current to the coil 8 with the lens focusing coil 7 bucking the coil 23 to provide a magnetic field which is nearly zero at the exit from the plasma expansion cup 21 which in turn provides for the optimum ion current from the device.

The ion source is enclosed in a suitable enclosure and the enclosure is evacuated to a pressure of 5x10 mm.

.Hg, for example, in a conventional manner.

potential planes to the magnetic 'has been operated at accelerating voltages up to kv.

and magnetic fields (as measured in the throat of the focusing coil 7 below the electrode 24) up to 5K gauss without any electron discharge, which was impossible to achieve with the prior device of the above patent with such a magnetic field. Ion currents of up to a total of 9-00 ma. and a H+ beam of up to 500 ma. have been obtained under these operating conditions and the ion beam was stable and well focused and of very good quality, thus providing a more concentrated beam at distant points of focus than heretofore possible because of the ability to utilize a more concentrated magnetic field as discussed above. V

The device of FIG. 3 can be modified, if desired, to provide a centrally apertured target cathode between the anode 22 and the coil 23 and insulated therefrom with the expansion cup 21 afiixed to the target cathode in the same manner as the device described in the report ORNL-3908, March 1966, pp. 103-104. The use of such a target cathode in the device of FIG. 3 provides an ion beam that is just as stable and well focused as the ion beam produced by FIG. 3 without the target cathode.

Such a modified device with a target cathode is useful for producing a H; ion beam.

The device of FIG. 4 illustrates a modification of the device of FIG. 3, wherein the accelerating region thereof- '19, 21, and 23 of FIG. 3. The other components of the device of FIG. 4, which are not shown, are identical to those utilized for the device of FIG. 3.

In FIG. 4, a plurality of centrally apertured accelerating electrodes 24' are provided between the cup 21' and a grounded electrode 32. The metallic member 33 is' electrically connected to the anode, target cathode, not shown, when such a cathode is utilized. Each of the electrodes 24' is connected to respective sources of high voltage, only one of such connections being shown, by means of a respective terminal 28," a lead 29, a connector 30, and a lead 31..The member 33 is connected to a 600-kv. source of potential, and the outer, middle, and inner electrodes 24' are connected to 450-kv., 300-kv;, and -kv. sources of potential,,r e-

spectively. A plurality of pumping slots 35 are provided to facilitate the evacuation of the accelerating region. Thus, the electrodes 24' of the ion source of FIG. 4 are each shaped to provide the desired relationship of equifield, and the arch of each prevents line of sight between the ion beam and any portion of the common supporting insulator '19.

The device of FIG. 4 has allof the advantages of the device of FIG. 3, and is useful as a high energy ion in-' jector for controlled thermonuclear reactor devices and for various accelerator systems.

This invention has been shown duoplasmatron-type ion source by way of illustration rather than limitation and it should be apparent that it is equally applicable with'other sources of ions and infields other than those described.

What is claimed is:

1. In an ion source device in an evacuated enclosure,

said device being provided with a heated electron emis- I not shown, or to the and described with a" a cooled intermediate ion collimating electrode disposed between said filament electrode and one side of said anode electrode and being spaced from and in axial alignment with said anode electrode and filament electrode, said intermediate electrode also enclosing said filament electrode, an ion extracting electrode disposed on the other side of said anode electrode in spaced relation and axially aligned therewith, an adjustable power supply connected to said filament electrode, an adjustable source of operating power connected across said anode electrode and filament electrode for establishing an arc discharge therebetween, an adjustable source of accelerating voltage connected across said anode electrode and extracting electrode, an electromagnetic source solenoid disposed about said filament electrode with a source of magnet power supply connected thereto, a plasma expansion cup aflixed to said anode electrode in axial alignment with said anode aperture and being positioned in the space between said anode electrode and extracting electrode, an electromagnetic solenoid coil encompassing said plasma expansion cup, an adjustable power supply means connected to said solenoid coil to provide a selected magnetic field, means for feeding gas at a selected, controlled rate into the space between said intermediate electrode and said anode electrode, an electromagnetic solenoid lens axially aligned with said electrodes and positioned beyond said extracting electrode for focusing the ion beam from said extracting electrode, means for energizing said solenoid lens such that its magnetic field bucks the magnetic field provided by said solenoid coil encompassing said expansion cup, and means for adjusting said power supplies to said source solenoid and to said solenoid coil to provide a near-zero magnetic field at the exit of said expansion cup, the improvement wherein there are provided (1) an extensive fiat plate, (2) a base plate supporting said plasma cup solenoid coil, said base plate being mounted in a recess of said extensive fiat plate such that said plates together form a flat electrode of the accelerating system, said ion extracting electrode being in the shape of an inverted shallow dish with respect to said flat electrode, and (3) a flat insulator supporting the side walls of said dish-like extracting electrode, said insulator also supporting said extensive flat plate, said shallow-dish extracting electrode being so designed in relation to said flat electrode such that there is no point of maximum potential along a magnetic field line, and (4) a centrally apertured, grounded electrode positioned between said extracting electrode and said solenoid lens, whereby electrons are drawn out of the region as they are formed to thereby prevent electron accumulation, electron oscillations and uncontrolled discharges, thus allowing said device to produce a well focused ion beam at relatively large values of accelerating voltages and magnetic fields.

2. The device set forth in claim 1, wherein said adjustable source of accelerating voltage is of a selected value in the range from 0100 kv., and the magnetic field in the throat of said focusing solenoid lens is of a selected value up to 5 kilogauss.

3. The device set forth in claim 1, wherein a plurality of additional ion extracting electrodes are provided in the acceleration region of said device, said additional extracting electrodes also being in the shape of inverted shallow dishes with respect to said flat electrode and being in spaced relation to each other and to said first ion extracting electrode, all of said extracting electrodes being so designed in relation to each other and to said flat electrode such that there is no point of maximum potential along any magnetic field line, and a plurality of selected high voltage sources connected to respective ones of said extracting electrodes to provide a plurality of respective accelerating steps in said accelerating region.

4. The device set forth in claim 3, wherein at least three extracting electrodes are provided, said flat electrod being connected to a high voltage source of about 600 kv., said high voltage sources connected to said respective extracting electrodes being respectively about 450 kv., 300 kv., and kv., said centrally apertured, grounded electrode positioned between the last of said extracting electrodes and said solenoid lens.

5. The device set forth in claim 2, wherein a centrally apertured, target cathode is positioned between said anode and plasma expansion cup with said cup being afiixed to said target cathode.

6. The device set forth in claim 3, wherein a centrally apertured, target cathode is positioned between said anode and plasma expansion cup with said cup being afiixed to said target cathode.

References Cited UNITED STATES PATENTS 3,238,414 3/1966 Kelley 313-230 X FOREIGN PATENTS 835,118 5/ 1960 Great Britain.

JAMES W. LAWRENCE, Primary Examiner. STANLEY D. SCHLOSSER, Examiner, 

1. IN AN ION SOURCE DEVICE IN AN EVACUATED ENCLOSURE, SAID DEVICE BEING PROVIDED WITH A HEATED ELECTRON EMISSIVE FILAMENT ELECTRODE, A COOLED ANODE ELECTRODE PROVIDED WITH AN ION EXTRACTION, CENTRALLY DISPOSED APERTURE SPACED FROM AND IN AXIAL ALIGNMENT WITH SAID FILAMENT ELECTRODE, A COOLED INTERMEDIATE ION COLLIMATING ELECTRODE DISPOSED BETWEEN SAID FILAMENT ELECTRODE AND ONE SIDE OF SAID ANODE ELECTRODE AND BEING SPACED FROM AND IN AXIAL ALIGNMENT WITH SAID ANODE ELECTRODE AND FILAMENT ELECTRODE, SAID INTERMEDIATE ELECTRODE ALSO ENCLOSING SAID FILAMENT ELECTRODE, AN ION EXTRACTING ELECTRODE DISPOSED ON THE OTHER SIDE OF SAID ANODE ELECTRODE IN SPACED RELATION AND AXIALLY ALIGNED THEREWITH, AN ADJUSTABLE POWER SUPPLY CONNECTED TO SAID FILAMENT ELECTRODE, AN ADJUSTABLE SOURCE OF OPERATING POWER CONNECTED ACROSS SAID ANODE ELECTRODE AND FILAMENT ELECTRODE FOR ESTABLISHING AN ARC DISCHARGE THEREBETWEEN, AN ADJUSTABLE SOURCE OF ACCELERATING VOLTAGE CONNECTED ACROSS SAID ANODE ELECTRODE AND EXTRACTING ELECTRODE, AN ELECTROMAGNETIC SOURCE SOLENOID DISPOSED ABOUT SAID FILAMENT ELECTRODE WITH A SOURCE OF MAGNET POWER SUPPLY CONNECTED THERETO, A PLASMA EXPANSION CUP AFFIXED TO SAID ANODE ELECTRODE IN AXIAL ALIGNMENT WITH SAID ANODE APERTURE AND BEING POSITIONED IN THE SPACE BETWEEN SAID ANODE ELECTRODE AND EXTRACTING ELECTRODE, AN ELECTROMAGNETIC SOLENOID COIL ENCOMPASSING SAID PLASMA EXPANSION CUP, AN ADJUSTABLE POWER SUPPLY MEANS CONNECTED TO SAID SOLENOID COIL TO PROVIDE A SELECTED MAGNETIC FIELD, MEANS FOR FEEDING GAS AT A SELECTED, CONTROLLED RATE INTO THE SPACE BETWEEN SAID INTERMEDIATE ELECTRODE AND SAID ANODE ELECTRODE, AND ELECTROMAGENTIC SOLENOID LENS AXIALLY ALIGNED WITH SAID ELECTRODES AND POSITIONED BEYOND SAID EXTRACTING ELECTRODE FOR FOCUSING THE ION BEAM FROM SAID EXTRACT- 